Switched-mode power converters are widely used to convert between differing DC voltages. FIG. 1 shows an example of a typical buck converter (used to provide a reduced voltage from a higher-voltage supply).
Transistor switches are used to alternately connect an inductor to a supply voltage and a ground, at a switching frequency fsw. The output of the inductor is connected to a capacitor and resistive load. The output voltage is connected to the terminal VSENSE, allowing closed-loop feedback control thereof. The target output voltage may be intentionally varied, as discussed in more detail below. In FIG. 1, a terminal VREF is depicted, which may be used to provide an analog voltage used to control VOUT.
Most DC-DC converter applications must support changes in load current (load transients). Some applications also require rapid changes in output voltage. In certain specialized applications, both rapid changes in output voltage and current are required, and further, the relative timing of these changes is constrained to support specific requirements of the load. For example, a mobile device may use the Global System for Mobile Communications (GSM) protocol to communicate with a base station. GSM uses time-division multiplexing, wherein each mobile device is allocated one or more time slots, during which it is the only mobile device transmitting on a given channel. In a default operating condition, used, for example, for voice communication, a mobile device is assigned one of eight possible time slots in which to transmit, and a constant-envelope phase modulation (Gaussian Minimum Shift Keying, or GMSK) is used. Data communications configurations, in which a linear modulation is employed, may use up to four consecutive timeslots. Further, it is also possible to have consecutive time slots with differing modulation. When the device is not transmitting, the power amplifier (PA) used to supply the final transmitted signal to the antenna is usually turned off to save power. The best overall efficiency is obtained by using a switched mode power supply to provide the supply voltage for the PA. Further, it may be desirable to set the regulated power supply to a low level or 0 when the PA is not in use.
In consequence, the switched mode power supply for the PA must supply a rapidly-changing load current at constant output voltage. The rate at which the RF power, and therefore the load current, increases during the beginning of the burst is set by the requirement that the transmitted power be very low outside the desired channel. An excessively rapid increase in output power results in radiated signals outside the desired band, due to the well-known property of Fourier transforms that rapid changes in the time domain correspond to wide bandwidth in the frequency domain.
FIG. 2 provides time lines that summarize typical control of an output ramp of a power amplifier. The graph 210 represents an example of output power versus time 250 near the slot boundary, taken to be at 0 microseconds. Also shown are limits on the allowed output power within specific time windows around the slot edge, 230 and 240, as prescribed in the relevant GSM standards. The graph 220 depicts the corresponding changes in current drawn by the power amplifier as a function of time, 260. About 15 microseconds before the beginning of the transmission slot, the PA is enabled and the voltage of the regulated supply is brought to the desired value (if it was lowered after the previous transmission). At about 13 microseconds before the slot edge, the RE power 250 is turned on and set temporarily at a low level. The power is then ramped up smoothly over a period of about 10 microseconds, after which the modulated transmission begins. A similar ramp is used to reduce the output power at the end of the slot. The current 260, depicted in a linear scale, rises rapidly over the corresponding times. The final power is fixed to within ±1 dB.
The current required by the power amplifier may change substantially depending on the load presented at the antenna. For example, a typical power amplifier designed for 2 Watts transmitted average power may require 1200-1600 mA supply current at 3.3 V when the antenna is well-matched. However, when a 10:1 mismatch occurs (as may result, e.g., when a user's hand is placed over the antenna), the current may increase to as much as 2700 mA. These excursions must be supported, but are relatively rare. An output inductor Lout large enough to supply the maximum current without encountering saturation of the magnetic core would be substantially larger and more expensive than required for normal operation.
Various approaches to the problem of supporting load transients in a switched DC-DC converter have been reported. In one approach, an auxiliary switched converter, for example using a smaller output inductor for faster response, is placed parallel with the main converter. A second switched converter requires additional switches and an additional output inductor, the latter being large enough to support the required saturation current, and thus does not provide the desired reduction in size relative to a high-saturation-current main output inductor. In another approach, a transistor or linear regulator drives the output node of the converter. It is necessary to ensure that the linear regulator controller and the switching converter controller do not interact in an undesired fashion, leading to oscillations and instability. Control approaches include adjustment of the switched converter current to drive the linear regulator current to 0, which is obviously unsuitable for solving the problem discussed above, since then the switched converter would eventually provide all the output current, and require a large output inductor. The linear and switched regulators may be used alternately, with one active only when the other is inactive. Obviously not helpful for reducing maximum switched current. Alternatively, conventional controls can be used for both the switched converter and linear regulator, but with the target voltage for the linear regulator set lower than that of the switched converter. Since the linear regulator can only pull the output voltage up, this results in a graceful transition between linear and switched converter control. However, this approach does not allow a constant-output-voltage operation with sustained controlled current sharing between the linear regulator or bypass FET and the switched converter.
It is desirable to have methods, apparatuses, and systems for providing additional current to the load of a switching regulator, without requiring switches and an output inductor be sized to support the maximum current that might be required.